The version I made to Instructablize is shaped a little differently, and the construction is better. Neither used a pattern, I drafted the pattern directly onto the fabric before cutting. If you have a pattern for a coat by all means use it, but if not I am including a brief description of how to draft this one. (Note that a coat with a collar presents difficulties for a reversible garment - if you're not that experienced yet I suggest not using a collared pattern.)
Step 1: A Note on Fabric Choice
Buy about 2 to 2-1/2 yards, unless you are very big or very small, or the fabric is very skinny. If you're using a commercial pattern, follow its suggestions, otherwise if you're not confident of your yardage estimate, you may want to draft a paper pattern first and lay it out to see how much fabric it will truly need.
Step 2: Drafting the Pattern
Divide your circumferential measurements (modified versions) by 8 and note down the results. This will be the approximate width of the four pattern pieces you will be drawing (you'll cut with the fabric folded in half so the total pieces to sew will be 8).
Now draw shapes as in the sketch below - see the notes for important details. You'll want to move a little bit of width from piece to piece to match your shape, but remember to keep the total values the same. For example, the hip measurements are usually mostly rear-end, so you'll want the back pieces to be a bit larger at the hip than the front pieces, to let the garment hang correctly.
(If you have a commercial pattern, you can skip this step and the next entirely.)
Step 3: Draft the Sleeve Pattern
- The length of your arm from the point of your shoulder to your wrist (or wherever you want the sleeve to come down to)
- The armscye you'd like - armscye means the seam that goes around your arm at the shoulder, to attach a sleeve to the bodice of a garment. Wrap the tape measure loosely around your shoulder and make some arm movements to ensure it's not too big or too small.
- The bicep - this is a highly variable measurement depending a lot on how tight or loose you want the garment. Try to measure what the sleeve ought to feel like, rather than the actual distance around your arm. You won't add any ease to this measurement.
- The wrist. Same notes apply here as to the bicep measurement.
Step 4: Draw the Pattern Onto the Fabric
Transfer your sketch onto the fabric using anything you like that will mark on your fabric: chalk, felt-tip pen, water-soluble pen, sharpie... If your fabric is white or very thin you may choose to draw your patterns onto paper first, newspaper taped together works well. Of course you can do that anyway if you don't like the idea of drawing on the fabric - but all the markings will be in between the two layers when you're done, and not visible from the inside or outside of the garment.
If your fabric is wide enough, you can exactly copy your sketch onto it, but you will probably have to move at least one of the pieces lower down.
I used red ink which showed up fine while I was working but has not come out well in the pictures, my apologies.
VERY IMPORTANT: make sure to leave at least an inch of room between each piece you draw, for seam allowance. (A commercial pattern may, depending on brand, have the seam allowance included.)
Step 5: Cutting Out the Body Pieces
Lay out your other piece of fabric and, using your first set as a pattern, cut that out as well. You should end up with four of each piece, two of fabric A and two of fabric B. Each pair cut from the same fabric should be mirror images.
Step 6: Body Seams
Step 7: Sleeves
If the lengths are off by more than an inch, you have a couple options. If the sleeve is too big, check if it is also big on your arm. You may be able to simply restitch the underarm seam, which will reduce the length of the armscye. Alternately, you can cut the armscye on the body a little bigger (be careful here, a small change goes a long way).
If the sleeve is too small, you may be able to fix it if the difference isn't too great, by changing the angle of the sleeve armscye. In the first picture below, you can see how there's only about a 15 degree angle off of horizontal for the sleeve caps; you can make this angle greater by cutting away fabric, at the cost of shortening the underarm length. You may, however, have to bite the bullet and re-measure and re-cut the sleeves.
Step 8: Second Shell
Most of the seams should be topstitched as you sew them, but there are two exceptions. The armscye seams should not be topstitched at all. Because of the shape of the seams, folding the seam allowances back on themselves doesn't work well; also it fills the sleeve cap out nicely if the seams are folded towards the sleeve.
The other exception is the center back seam. Remember to stop stitching at the tail split - match this up to the first shell. You'll topstitch this one once both shells are sewn together, to stabilize them.
You will find it somewhat annoying to topstitch the sleeve seams, as you have to work inside the tube. This can be done by scrunching the fabric up as you sew, but you can also skip topstitching those if you want.
Step 9: Pin the Shells Together
Start pinning at the back neck and work your way around. Pin every 6 inches or so. As you get down the sides, hold the two pieces up every so often to ensure they are hanging correctly - get your hands into the shoulder seams so they're hanging from the same point. This is kind of annoying with the sleeves all bundled up inside, so take care.
It doesn't matter if the edges don't match exactly around the tails, it is more important that the two shells hang at the same length. It may take a little stretching and a couple of tries to get the hang correct, but better now than after you've sewn it together! Don't worry about the bottoms of the seams lining up either, they will probably be a little off.
Step 10: Stitch the Shells Together
Your edges will not match up exactly as you get lower down on the coat, but follow the pins and make the curve a smooth one as you go. Also the fabrics will meet each other differently when lying flat than when they were hanging, so trust your pinning and follow its lead. It may be helpful to trim the longer pieces off before you sew, if the underneath layer is shorter.
Step 11: Turn and Try
Try it on to make sure the edges hang well. If there are any problems it's not too hard to fix them at this point, just mark the problem area, turn it inside out again, pick out the stitches and resew (I do not think I have ever made a garment where I didn't have to pick out some stitches).
When you're satisfied, turn the points of the tail wrong side out one last time and trim the seam allowance to reduce bulk.
Step 12: Sleeve Hem Stitching
First turn the coat right side out and insert one shell's sleeves inside the other's as if you were wearing it normally. Fold the sleeve hems together at the underarm seam, as if the hem were already sewn together from the inside, and put one pin in that one spot in each sleeve.
Now turn the whole thing back inside out through the tails and see how the pinned-together sleeves come out! They are facing each other lengthwise, like two pieces of tube held end to end! Pin the sleeve hems the rest of the way around, and sew each seam.
Now turn the coat right-side out again - here's what the turning right-side-out looks like:
A slight digression. There's a branch of mathematics called topology, which deals with the properties of shapes that don't change when you stretch them. It has a lot of applicability to sewing. A common thought problem is, can a torus (a hollow doughnut shape) be turned inside out through a hole in its side? It can, and it's still a torus; but the new torus's center hole is not the same hole as the original one. This is not really explainable in words, you have to try it to understand - and the reason I brought it up is that this is almost exactly what is being done here, with a double torus - the two sleeves of the coat form this shape.
If none of that made sense, don't worry, it won't affect the sewing process!
Step 13: Center Back Topstitching
Lay the coat out smoothly and pin the center back seams together as shown in the pictures. Carry the pins all the way down to the ends of the tails.
Now stitch next to the seamline on each side, just like the other topstitching, only going through both layers. Backstitch at the neck and at the ends of the tails. (You can see the topstitching in the photos if you look closely.)
You are done! Wear either side out, depending on your mood. You will be very warm with all those fuzzy layers, and of course, super stylish.